HIGH RESOLUTION ELECTROMECHANICAL MAPPING OF THE HEART We have continued to develop the hardware, MR pulse sequences and data analysis software to perform high resolution mechanical function maps while simultaneously recording electrical activation maps from an electrode "sock" surrounding the heart. We have also developed the hardware to perform endocardial mapping simultaneously with the epicardial mapping using a basket electrode. The electrical recording system has been made MR compatible so that the MR image quality is not degraded by the presence of the sock around the heart. Coupled electromechanical activation maps are being generated by registration of the electrical maps with the mechanical maps generated from MRI tagging studies. The effect of ischemia on the electromechanical can now be studied in detail, perhaps leading to the ability to precisely distinguish viable from non-viable tissue. An MR marker system has been added to the protocol to allow precise registration of the digitized position of the markers, and their location in the MR images. Diffusion MR is now being used to map the myocardial fiber angle with sub-millimeter resolution over the entire heart to aid in modeling the progression of the electrical activation. We have also begun to measure the blood flow pattern in the cardiac chambers associated with different activation patterns. A new parameter has been derived to describe the asynchrony in mechanical activation associated with LBBB and heart failure, and the parameter coorelates well with improved function during resynchronization therapy.